This chapter is from the book

This chapter is from the book

Computers know nothing about images, or tone, color, truth, beauty, or art. They're just very complicated adding machines that crunch numbers. Every piece of data we store on a computer is comprised of numbers. All the commands we send to the computer are translated into numbers. Even this text that I'm typing is made up of numbers.

Fortunately, you don't have to learn hexadecimal or binary math to use Photoshop—we're living math-challenged proof of that—but if you want to put Photoshop under your control, rather than flailing around and occasionally getting good results by happy accident, you do need to understand the basic concepts that Photoshop and other image editors use to represent photographs using numbers.

We'll keep it simple and equation-free (the computer does the math for you), but unless you like heavily pixellated output and wildly unpredictable color shifts, you really want to understand the essential lessons about images that we lay out in this chapter.

Pixels and Paths

When you get down to the nitty-gritty, there are essentially two ways to make computers display pictures. In Photoshop terminology, the distinction is between pixels and paths. Other terms you may hear are "raster" (rasters are rows or lines, not reggae) and "vector." We call the stuff made up of pixels "images" and the stuff made of vectors "artwork."

Pixel-based images

Images are simply collections of dots (we call them pixels or sample points) laid out in a big grid. The pixels can be different colors, and the number of pixels can vary. No matter what the picture is—whether it's a modernist painting of a giraffe or a photograph of your mother—it's always described using lots of pixels. This is the only way to represent the fine detail and subtle gradations of photorealistic images.

Just about every image comes from one of three sources: capture devices (such as scanners, digital cameras, or video cameras), painting and image-editing programs (such as Photoshop), or screen-capture programs (like Snapz Pro, the operating system, and a host of others). If you create a document with any of these tools, it's an image.

Vector artwork

Vector artwork, also known as object-oriented graphics, is both more complex and more simple than a pixel-based image. On the one hand, instead of describing a rectangle with thousands (or millions) of dots, vector graphics just say, "Draw a rectangle this big and put it here." Clearly, this is a much more efficient and space-saving method for describing some kinds of art. Vector graphics can include many different types of objects, including lines, boxes, circles, curves, polygons, and text blocks. And all those items can have a variety of attributes—line weight, type formatting, fill color, graduated fills, and so on.

To use an analogy, vector graphics are like directions saying, "Go three blocks down the street, turn left at the 7-11, and go another five blocks," while pixel-based images are more like saying, "Take a step. Now take another step. And another...." At its core, Photoshop is a tool for working with pixels, but each iteration has offered more support for incorporating vector elements that retain their object-oriented characteristics, such as shapes or type, and you can also use vector elements as selections and masks on pixel-based images.

Outside Photoshop, vector graphics come from two primary sources: drawing programs (such as Adobe Illustrator), and computer-aided design (CAD) programs. You might also get vector artwork from other programs, such as a program that makes graphs.

Words, Words, Words

While terminology might not keep you up at night, we in the writin' business have to worry about such things. In fact, one of our first controversies in writing this book concerned the term bitmap.

Bruce maintains that, strictly speaking, bitmaps are only black-and-white images. This is how Photoshop uses the term. He prefers to describe images made up of colored dots as raster images (the word "raster" refers to a group of lines—in this case, lines of pixels—that collectively make up an image). David thinks that only people who wear pocket protectors (some of his best friends do) would use the word "raster." For years we compromised by using the term "bitmapped images"—which resulted in a lot of sentences with at least one wooden leg. So in this edition, we call documents comprised of pixels "images" and documents comprised of vectors "artwork."

Another problem we've encountered is what to call all those little dots in an image. As we mentioned earlier, when we talk about points in an image, we like to call them pixels, samples, or sample points.

The phrase "sample points" comes from what a scanner does: it samples an image—checking what color or gray value it finds—every 300th of an inch, every 100th of an inch, or whatever. But nowadays, many images are captured with a digital camera rather than being scanned, which makes the concept of sample points more questionable. Pixel is a more generic term because it specifies the smallest "picture element" in an image. Some people still call pixels pels. They may not wear pocket protectors, but they've almost certainly had an unnaturally close relationship with an IBM mainframe somewhere in their past.

When we talk about scanning an image in, or printing an image out, we talk about samples or pixels per inch (while samples is closer to reality, everyone we know uses the latter, or ppi); and when we talk about the resolution of a bitmapped image saved on disk, we just talk about the total number of pixels. Note that many people use "dots per inch" (dpi) for any and all kinds of resolution. We reserve the term "dots per inch" for use when speaking of printers and imagesetters, which actually create dots on paper or film.

We use the term "pixels" for one other thing: screen resolution. But to be clear, we always try to specify "screen pixels" or "image pixels."

Crossing the line

The distinction between images and artwork occasionally gets fuzzy, because vector artwork can include pixel-based images as objects in their own right. For instance, you can put a digital camera capture into an Adobe Illustrator illustration. The image acts like an object on the page, much like a rectangle or oval. You can rotate it, warp it, and scale it, but you can't go into the image and change the pixels.

A vector artwork file may include a pixel-based image as its only object. In this situation, the file is an image that you can open for editing in a painting or pixel-editing application. Photoshop's PDF (Portable Document Format) files are good examples of this. While PDF is typically a vector file format, you can create a pixel-only PDF in Photoshop.

Just to round out the confusion, Photoshop lets you include vector elements in pixel-based images, either as stand-alone objects (like text) or as clipping paths. A clipping path in an image is invisible; it acts as a cookie cutter, allowing you to produce irregularly shaped images such as the silhouetted product shots you often see in ads (see "Clipping Paths" in Chapter 12, Essential Image Techniques).